1. Field of the Invention
The present invention relates to a method for enhanced power saving on Distributed Coordinated Function (DCF) based wireless networks. Specifically, the invention relates to a method which divides a beacon transmitting section in an Announcement Traffic Indication Message (ATIM) window period, defined by IEEE standard 802.11, according that there is a data packet to be sent or not and allows a terminal that has received the beacon to go to a doze state immediately according to beacon reception timing, thereby preventing unnecessary power consumption.
2. Description of the Background
A power saving mechanism in DCF, defined in IEEE 802.11, will be explained hereinafter with reference to FIG. 1.
A basic approach for saving energy in IEEE standard 802.11 is to periodically convert the state of a radio terminal to a doze mode. The radio terminal being in the doze state changes its state to an awake state in order to receive a beacon or Traffic Indication Map (TIM) or transmit MAC Service Data Unit (MSDU). To periodically change the states of radio terminals, synchronization must be carried out for all of the radio terminals. In a Point Coordinated Function (PCF) mode, Point Coordinator (PC) as an operator performs global synchronization. In DCF, however, all of radio terminals carry out synchronization through beacon because there is no global synchronization operator in DCF.
That is, beacons are generated in such a manner that they are distributed in all of radio terminals of a network in DCF. Each radio terminal has its timing synchronization function (TSF) timer in order to synchronize with a beacon cycle. Furthermore, each beacon includes its time stamp. All the radio terminals that have packets to be send during a beacon interval transmit their beacons using standard back-off algorithm.
In the case where multiple radio terminals transmit their beacons within one beacon interval, the beacon of the radio terminal that has sent it for the first time is selected and the other beacons sent by the remaining radio terminals are cancelled. In FIG. 1, for example, the beacon of node A is selected and beacons of the remaining radio terminals are cancelled. When a specific beacon is selected, all of other radio terminals adjust their timer to timestamp of the selected beacon so that all of the radio terminals can accomplish synchronization. At this time, beacon transmission is carried out in an awake-period of the beacon interval.
In the meantime, as shown in FIG. 2, IEEE standard 802.11 defines an ATIM window as a period during which radio terminals are awake in the beacon interval. During the ATIM window period, all the radio terminals can exchange beacons and ATIM/ACK packets with one another. All of the radio terminals must be converted into the awake state during the ATIM window period in order to achieve synchronization and to receive packet transmission announcements.
A packet transmission announcement is made through an ATIM packet. When a radio terminal A (node A) has a data packet to send to a radio terminal B (node B), the radio terminal A stores the data packet in a buffer and performs synchronization through beacon, and then transmits the ATIM packet to the terminal B. The terminal B receives the ATIM packet sends an ACK packet in answer to the ATIM packet. The radio terminal B, which has exchanged the ATIM packet and ACK packet with the radio terminal A, during the ATIM window period, does not change its state to the doze state after the ATIM window period but carries out data communication while exchanging the data packet and ACK signal with the radio terminal A. Other radio terminal C (node C), is converted to the doze when the ATIM window is finished.
As described above, in a Power Saving Mode (PSM) of IEEE standard 802.11, each radio terminal has ATIM window period with a fixed length and a doze period. However, this has several inefficient factors in terms of energy use. Firstly, even in the case where there is no traffic on a network, radio terminals are converted into the awake state and consume energy during the ATIM window period because the length of the ATIM window period is fixed. Secondly, in an IEEE standard 802.11 PSM technique, corresponding radio terminals should be awake until the beacon interval is finished even after data transmission is completed, resulting in unnecessary waste of energy.